Effect of lignin content and structure on the anatomical, physical and mechanical properties of genetically engineered aspen trees

The directed modification of specific traits of trees through genetic engineering provides opportunities for making significant genetic improvements to wood properties in a matter of years instead of extended time frames required for traditional natural selection. An attractive target of forest-tree engineering is the modification of lignin content and its structure which present potential advantages including improved pulping efficiency, lower chemical and energy consumption, and reduced environmental impacts. However, decrease in lignin content and changes in lignin structure could lead to the modifications of wood characteristics that are critical for solid wood.Wild-type and transgenic quaking aspen (Populus tremuloides Michx.) with reduced lignin content, increased syringyl to guaiacyl (S/G) ratio, and both reduced lignin content and increased S/G ratio were investigated in four studies.In the first study, diameter growth and cell morphology of one-year-old transgenic aspen were investigated using quantitative wood anatomy and fiber quality analysis techniques. Similar radial growth and quantitative anatomical properties were observed between the genetic group with reduced lignin content and the wild-type. The genetic group with increased S/G ratio had lower diameter growth, lower vessel lumen diameter, but more numerous vessels. The combined effect of lignin content and structural changes on radial growth and cell morphology seems to be more complex and gave inconsistent results.In the second study, dynamic modulus of elasticity (MOE) using a nondestructive technique (Fakopp Microsecond Timer) and static MOE using micromechanical testing method were investigated for 2.5-year-old transgenic aspen clones. Result showed that a reduction in the lignin content reduce both the dynamic and static MOE. Increase in the syringyl to guaiacyl ratio results in only a slight decrease in the dynamic MOE and static MOE. The combined influence of lignin content and syringyl to guaiacyl ratio changes shows the most obvious negative effect on both the dynamic MOE and static MOE.In the third study, a dynamic mechanical analyzer (DMA) in static bending mode was used to determine the elastic modulus of 2.5-year-old wild-type and transgenic aspen submerged in water and in ethylene glycol. DMA MOE values were compared to those assessed by nondestructive evaluation and micromechanical testing in previous study. All of the measured elastic moduli showed the same trend and detected similar differences across the genetic groups. DMA measurements showed notably lower MOE values than the other techniques.In the fourth study, the in situ lignin glass transition of one-year-old transgenic aspen was investigated using dynamic mechanical analysis and rheometer. Results suggested that an increase in the S/G ratio did not affect the glass transition temperature but a reduction in lignin content decreased the softening temperature of transgenic wood which has practical implication and would be beneficial to pulp and ethanol production.This research revealed that a reduction in lignin content had a more severe negative effect on wood properties than an increase in S/G ratio. However, diameter growth of transgenic trees with increased S/G ratio was lower. The combined effect of reduced lignin content and increased S/G ratio on wood characteristics was inconsistent therefore it needs further investigation. Although the reduced lignin content genetic group has inferior mechanical properties and cannot be considered for solid and structural applications, the lower lignin content and glass transition temperature make it suitable for pulp and ethanol production, and for composite manufacture.